1. Discrepancies previously observed between the spectra of butadiene and 2, 3-dimethylbutadiene around the second valence transition have been demonstrated to arise from a thermally induced enhancement of this forbidden valence transition. 2. The barrier to methyl torsion in 2, 3-dimethylbutadiene has been shown to be due predominantly to the out-of-plane pi electron orbitals of the molecule. 3. The valence spectra of cis and trans hextrienes have been studied. In each molecule, the allowed valence transition is lowest in energy. The first singlet and triplet transitions occur at the same energies in the two isomers. both the second singlet and triplet transitions are higher in energy in the trans molecule than in the cis molecule.